As well known, while various electronic devices tend to minimization and light weight, they are also developing to multifunction and high speed. Electromagnetic waves with various frequencies will be generated and radiated during operation of these electronic devices, resulting in cross interference. Thus, it is an effective measure for shielding the electronic devices to prevent the interference of electromagnetic waves. This may not only prevent the incident electromagnetic wave from interfering with electronic devices in operation, but also effectively prevent the electromagnetic wave radiated during operation of these devices from interfering with other electronic devices in operation. Under such circumstances, the requirement for application of electromagnetic wave shielding in the flexible printed circuit (FPC) used for assembly of electronic devices is increasing.
At present, methods for shielding electromagnetic waves in FPC mainly comprise:
Copper foil: in this method, copper foil is hot-pressed on FPC products for shielding electromagnetic wave. The copper foil has a better shielding effect and flexibility. However, its thickness is larger, and hot pressing should be performed more than one time. Thus, this method has a long production period and higher cost.
Silver slurry: in this method, a layer of conductive coating, such as silver slurry, is uniformly coated on the surface of FPC products by printing and the like, for shielding electromagnetic wave. This method has advantages of low cost and good flexibility. But it has disadvantages of poor shielding effect, coating peeloff easily, requirement of printing, hardening and the like, long production period.
Silver foil: in this method, a new type of shielding material, i.e. silver foil, which was developed by Japanese TATSUTA company, is used. It includes protective film layer, insulation layer, metal foil layer, anisotropic conductive adhesive layer and release layer, and has excellent shielding effect and flexibility. Furthermore, it has advantages of fewer pressing times, short production period and high production efficiency, and is widely used in FPC in recent years.
Additionally, various electronic components, such as IC, resistor and capacitor, are mounted on a FPC. The insertion portions of the FPC should have a certain level of strength and stiffness to facilitate the assembly of electronic components. This needs to attach a reinforcement plate on these portions of the FPC. The typical reinforcement plates include: epoxy resin laminated plate with glass fiber as reinforcement material, such as FR4; shaping parts of heat resistance resin, such as PI reinforcement plate; aluminum plate; stainless steel plate; or composite reinforcement materials with a stereo shape by forging these materials in molds.
The reinforcement plate is attached by a layer of adhesive. The adhesive is mainly composed of thermosetting type and pressure sensitive type. High temperature and high pressure applied by hot pressing and baking processes are needed for thermosetting adhesive to complete its adhesive effect. Thus, this involves a long operation period and a higher cost. But only pressure is needed for pressure sensitive adhesive to provide this adhesive with a level of anti-peeling strength. Therefore, it has advantages of simple operation, short period and low cost, but its anti-peeling strength is a little lower.
With the minimization of electronic products, various electronic components tend to develop to miniature and superfineness. The FPC which carries various electronic components also tends to develop to miniature, high density and multifunction. Recently, some products require an effective bonding between the reinforcement plate and the shielding material. As shown in FIG. 1, the bonding method is as follows: a pressure sensitive double faced adhesive tape 2 is attached onto a reinforcement plate 1, to bond the tape 2 and the reinforcement plate 1 together by roll pressing. At the same time, the other side of the tape 2 is covered by a layer of release paper. The release paper, which covers the other side of the tape 2, is removed before the reinforcement plate 1 is attached to a silver foil 3, and then the reinforcement plate 1 is attached to the silver foil 3 by means of the tape 2. After adhesion, pressure is applied onto the reinforcement plate 1 to bond the tape 2 and the silver foil 3 together. However, the silver foil can not bond tightly to the reinforcement plate by the tape 3 during this bonding process, since the silver foil has a very smooth layer of protective film. Accordingly, the anti-peeling strength can not meet the designed application requirement.